Laser diode and method for fabricating same

ABSTRACT

A laser diode and method for fabricating same, wherein the laser diode generally comprises an InGaN compliance layer on a GaN n-type contact layer and an AlGaN/GaN n-type strained super lattice (SLS) on the compliance layer. An n-type GaN separate confinement heterostructure (SCH) is on said n-type SLS and an InGaN multiple quantum well (MQW) active region is on the n-type SCH. A GaN p-type SCH on the MQW active region, an AlGaN/GaN p-type SLS is on the p-type SCH, and a p-type GaN contact layer is on the p-type SLS. The compliance layer has an In percentage that reduces strain between the n-type contact layer and the n-type SLS compared to a laser diode without the compliance layer. Accordingly, the n-type SLS can be grown with an increased Al percentage to increase the index of refraction. This along with other features allows for reduced threshold current and voltage operation.

This application is a divisional application from, and claims thebenefit of, U.S. patent application Ser. No. 11/600,604 to Chakrabory etal., also entitled “Laser Diode and Method for Fabricating Same.”

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to laser diodes, and more particularly to nitridebased semiconductor laser diodes and methods for fabricating same.

2. Description of the Related Art

A laser is a device that produces a beam of coherent light as a resultof stimulated emission. Light beams produced by lasers can have highenergy because of their single wavelength, frequency, and coherence. Anumber of materials are capable of producing a lasing effect and includecertain high-purity crystals (such as ruby), semiconductors, certaintypes of glass, certain gasses including carbon dioxide, helium, argonand neon, and certain plasmas.

More recently there has been increased interest in lasers made ofsemiconductor materials. These devices typically have a smaller size,lower cost, and have other related advantages typically associated withsemiconductor devices. Semiconductor lasers are similar to other lasersin that the emitted radiation has spacial and temporal coherence, andlike other lasers, semiconductor lasers produce a beam of light that ishighly monochromatic (i.e. of narrow bandwidth) and is highlydirectional. Overall, semiconductor lasers provide very efficientsystems that are easily modulated by modulating the current directedacross the devices. Additionally, because semiconductor lasers have veryshort photon lifetimes, they can be used to produce high-frequencymodulation.

One type of semiconductor laser diode is referred to as an edge emittinglaser where the stimulated emission is from the side surface or edge ofthe laser diode. These devices typically have epitaxial layers in theform of waveguiding or reflective elements (cladding layers) with alight generating active region between the reflective elements.Additional layers can be included between the reflective elements toform a laser cavity. The edges of the laser diode can be cleaved duringmanufacturing to form edge reflective surfaces. A total reflectivity(TR) material can cover one edge, and an anti reflectivity (AR) materialcan cover the opposite edge. Light from the active region is reflectedbetween the edges and within the cavity by the reflective elements, withstimulated emission emitting from the edge with the AB material.

A known characteristic of laser diodes (and light emitting diodes) isthat the frequency of radiation that can be produced by the particularlaser diode is related to the bandgap of the particular semiconductormaterial. Smaller bandgaps produce lower energy, shorter wavelengthphotons, while wider bandgaps produce higher energy, shorter wavelengthphotons. One semiconductor material commonly used for lasers is indiumgallium aluminum phosphide (InGaAlP), which has a bandgap that isgenerally dependant upon the mole of atomic fraction of each elementpresent. This material, regardless of the different element atomicfraction, produces only light in the red portion of the visiblespectrum, i.e., about 600 to 700 nanometers (nm).

Laser diodes that produce shorter wavelengths not only produce differentcolors of radiation, but offer other advantages. For example, laserdiodes, and in particular edge emitting laser diodes, can be used withoptical storage and memory devices (e.g. compact disks (CD) digitalvideo disks (DVD), high definition (HD) DVDs, and Blue Ray DVDs). Theirshorter wavelength enables the storage and memory devices to holdproportionally more information. For example, an optical storage devicestoring information using blue light can hold approximately 32 times theamount of information as one using red light, using the same storagespace. There are also applications for shorter wavelength laser inmedical systems and projection displays. This has generated interest inGroup-III nitride material for use in laser diodes, and in particulargallium nitride (GaN). GaN can produce light in the blue and ultraviolet (UV) frequency spectrums because of its relatively high bandgap(3.36 eV at room temperature). This interest has resulted indevelopments related to the structure and fabrication of Group-IIInitride based laser diodes [For example see U.S. Pat. Nos. 5,592,501 and5,838,786 to Edmond et al].

Group-III nitride laser diodes can require relatively high thresholdcurrents and voltages to reach laser radiation because of optical andelectrical inefficiencies. These elevated current and voltage levels canresult in heat being generated during laser diode operation. In certainapplications, laser diodes are driven by a pulsed signal that results inpulsed laser light being emitted from the laser diode. The heatgenerated within the laser diode typically does not present a problemduring pulsed laser diode operation because the laser diode has theopportunity to cool during the lows of the signal. For other importantapplications, however, it can be desirable to drive the laser diode witha continuous wave (CW). CW operation is particularly applicable tooperation with optical storage devices that can require a continuouslight source for data storage and retrieval. Driving many currentGroup-III based laser diodes with a CW having the threshold current andvoltage necessary for laser emission can result in heating that candamage or destroy the laser diode. Heat sinks or other coolingmethods/devices can be employed to reduce operating heat within theselaser diodes, but the methods/devices can increase the cost andcomplexity of the devices and can require additional space.

SUMMARY OF THE INVENTION

The present invention is generally directed to laser diode epitaxialstructure having improved operating characteristics and improvedreliability, and methods for fabricating the epitaxial structures. Theimproved operating characteristics include operation as reduced currentand voltage thresholds, which allow for efficient operation at reducedtemperature.

One embodiment of a laser diode according to the present inventioncomprises an active region sandwiched between first and secondwaveguiding elements, and a compliance layer. The first waveguidingelement is on the compliance layer with the compliance layer reducingstrain between the first waveguiding element and other laser diodelayers. The first waveguiding element has a higher index of refractionthan a first waveguiding element in a similar laser diode without thecompliance layer.

One embodiment of a Group-III nitride laser diode according to thepresent invention comprises an InGaN compliance layer on a GaN n-typecontact layer and an AlGaN/GaN n-type strained super lattice (SLS) onsaid compliance layer. An n-type GaN separate confinementheterostructure (SCH) is on said n-type SLS and an InGaN multiplequantum well (MQW) active region is on the n-type SCH. A GaN p-type SCHon the MQW active region, an AlGaN/GaN p-type SLS is on the p-type SCH,and a p-type GaN contact layer is on the p-type SLS. The compliancelayer has an In percentage that reduces strain between the n-typecontact layer and the n-type SLS compared to a laser diode without thecompliance layer.

One embodiment of a method according to the present invention forfabricating a Group-III nitride laser diode comprises growing an n-typecontact layer on a substrate at a temperature within a first growthtemperature range. An n-type compliance layer is grown on said n-typecontact layer at a temperature within a second growth temperature rangethat is lower than said first growth temperature. An n-type waveguidingelement is grown on said compliance layer at a temperature within saidfirst growth temperature range, said waveguiding element grown with ahigher index of refraction compared to laser diodes without saidcompliance layer. An n-type separate confinement heterostructure (SCH)on said n-type waveguiding element, an active region is grown on saidn-type SCH, a p-type SCH is grown on said active region, and a p-typewaveguiding element is grown on said p-type SCH.

These and other further features and advantages of the invention will beapparent to those skilled in the art from the following detaileddescription, taken together with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is sectional view of one embodiment of a laser diode according tothe present invention at fabrication step where it has a contact layeron a substrate;

FIG. 2 is a sectional view of the laser diode in FIG. 1, with compliancelayer and cap layer on the contact layer;

FIG. 3 is a sectional view of the laser diode in FIG. 2 with astrained-layer supperlattice (SLS) on the cap layer;

FIG. 4 is a sectional view of the laser diode in FIG. 3 with a separateconfinement heterostructure (SCH) layer on the SLS and an undoped layeron the SCH layer;

FIG. 5 is a sectional view of the laser diode in FIG. 4 with a multiplequantum well (MQW) active region on the undoped layer;

FIG. 6 is a sectional view of the laser diode in FIG. 5 with a blockinglayer on the MQW active region;

FIG. 7 is a sectional view of the laser diode in FIG. 6 with a SCH layeron the blocking layer;

FIG. 8 is a sectional view of the laser diode in FIG. 7, with a SLS onthe SCH layer;

FIG. 9 is a sectional view of the laser diode in FIG. 8, with a contactlayer on the SLS;

FIG. 10 is a sectional view of one embodiment of a laser diode accordingto the present invention with contacts;

FIG. 11 is a sectional view of another embodiment of a laser diodeaccording to the present invention with contacts; and

FIG. 12 is a sectional view of still another embodiment of a laser diodeaccording to the present invention with contacts.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides high reliability, high output powernitride-based laser diode characterized by reduced threshold currentdensities and reduced threshold voltages. The present invention isparticularly applicable to continuous wave operation laser diodes,although it can also be used in pulsed wave operation laser diodes. Thereduced current and voltage thresholds result in lower heat build-up inthe laser diode during operation, which in turn reduces that likelihoodof damage or destruction of the device due to overheating. The presentinvention is also directed to methods for fabricating laser diodes withthese characteristics, and although the present invention is generallydirected to nitride-based laser diodes it is understood that it can alsobe applied to laser diodes made of other material systems.

The improved threshold and current characteristics result in the laserdiode operating at lower temperature without the need for external heatmanagement elements, such as heat sinks. The improved laser diodesaccording to the present invention can operate at many differentthreshold currents and voltages, with preferred laser diodes operatingwith a threshold current of less than 5 kA/cm² and threshold voltageless than 5 volts (V). One embodiment of a laser diode according to thepresent invention has threshold current in the range of 2-4 kA/cm² andthreshold voltage in the range of 4-5V, providing an output power of atleast 25 mW. These lower threshold voltages and currents are a factor inlowering the laser diode's operating temperature.

The improved threshold current and voltage characteristics are realizedby improving the waveguiding within the laser diode, such as byimproving the guiding efficiency of the waveguiding elements. Each ofthe waveguiding elements can comprise waveguide cladding layers in theform of strained-layer superlattice (SLS). By increasing the index ofrefraction of the layers, guiding efficiency can be improved and lessloss is experienced as light from the laser diode's active region isguided by the waveguiding elements. With less loss, stimulated emissioncan be achieved with lower threshold currents and voltages.

In one embodiment according to the present invention, the laser diode ismade from the Group-III nitride material system and in particular ismade from aluminum gallium nitride and gallium nitride material(AlGaN/GaN). As used herein, the term “Group III nitride” refers tothose semiconducting compounds formed between nitrogen and the elementsin Group III of the periodic table, usually aluminum (Al), gallium (Ga),and/or indium (In). The term also refers to ternary and quaternarycompounds, such as AlGaN and AlInGaN. As well understood by those inthis art, the Group III elements can combine with nitrogen to formbinary (e.g., GaN), ternary (e.g., AlGaN and AlInN), and quaternary(e.g., AlInGaN) compounds. These compounds all have empirical formulasin which one mole of nitrogen is combined with a total of one mole ofthe Group III elements. Accordingly, formulas such as Al_(x)Ga_(1-x)N,where 0≦x≦1, are often used to describe them. In Group-III nitride laserdiodes, the index of refraction of at least one of the waveguideelements is increased by arranging the laser diode so that aluminum (Al)mole fraction in the waveguiding element is increased.

In one embodiment according to the present invention, the waveguideelements can be fabricated with high Al content waveguide elements byemploying a compliance layer to provide strain relief. Without thisstrain relief there is a danger of cracking and other forms ofdegradation when growing high Al content Group-III nitride (AlGaN/GaN)waveguiding elements. Different compliance layers can be used with oneembodiment according to the present invention comprising a lowtemperature (LT) high indium (In) containing indium gallium nitride(InGaN) compliance layer, with an In percentage between 8 and 15 percent(%). Because of the reduced strain with other device layers as a resultof the compliance layer, the waveguide cladding layers can have an Alcontent in the range of 15 to 20%, although other AL percentages canalso be used. The higher AL content results in a larger difference inindex of refraction with the active region which allows for thewaveguide cladding layers to provide improved waveguiding.

Laser diodes according to the present invention can also have otherimprovements to allow for improved operating characteristics. Some ofthese include having the quantum well interface roughness reduced byadopting slower growth rate for the device n-type separate confinementheterostructure (SCH). Accordingly, the n-type SCH layer provides asmooth and uniform epitaxial layer upon which the quantum well activeregion can be formed. The p-contact resistance, and as result thethreshold voltage, can also be reduced by employing a thicker andheavily doped p-type GaN cap layer and by reducing the doping level ofthe layers in the n-type SLS structure. Laser diodes according to thepresent invention can also comprise a high temperature (HT) growthp-type SCH layer and a low temperature (LT) growth p-SLS layer. Thep-type SCH is grown at high temperature to assist Mg diffusion back intothe p-AlGaN cap.

The present invention is described herein with reference to certainembodiments but it is understood that the invention can be embodied inmany different forms and should not be construed as limited to theembodiments set forth herein. It is also understood that when an elementor component is referred to as being “on”, “connected to” or “coupledto” another layer, element or component, it can be directly on,connected to or coupled to the other layer element or component, orintervening elements may also be present. Furthermore, relative termssuch as “inner”, “outer”, “upper”, “above”, “lower”, “beneath”, and“below”, and similar terms, may be used herein to describe arelationship of one component or element to another. It is understoodthat these terms are intended to encompass different orientations of thedevice in addition to the orientation depicted in the figures.

Although the terms first, second, etc. may be used herein to describevarious layers, elements, components and/or sections, these layers,elements, components, and/or sections should not be limited by theseterms. These terms are only used to distinguish one layer, element,component, or section from another. Thus, a first layer, element,component, or section discussed below could be termed a second element,component, or section without departing from the teachings of thepresent invention.

Embodiments of the invention are described herein with reference tocross-sectional view illustrations that are schematic illustrations ofidealized embodiments of the invention. It is understood that many ofthe layers will have different relative thicknesses compared to thoseshown and that the laser diodes will have different shapes. Further,variations from the shapes of the illustrations as a result, forexample, of manufacturing techniques and/or tolerances are expected.Embodiments of the invention should not be construed as limited to theparticular shapes of the regions illustrated herein but are to includedeviations in shapes that result, for example, from manufacturing. Aregion illustrated or described as square or rectangular will typicallyhave rounded or curved features due to normal manufacturing tolerances.Thus, the regions illustrated in the figures are schematic in nature andtheir shapes are not intended to illustrate the precise shape of aregion of a device and are not intended to limit the scope of theinvention.

Referring now to the drawings and in particular FIGS. 1-9, a laser diode10 according to the present invention is shown at different steps in thefabrication process. A single laser diode 10 is shown, but it isunderstood that more typically many laser diodes are fabricated on asingle substrate wafer, with the devices then being separated intoindividual devices using known processes, such as scribe and breakprocesses.

Laser diodes can be formed on a substrate wafer using known fabricationprocesses such as growth in a reactor by metalorganic chemical vapordeposition (MOCVD). Under some circumstances carbon can form in thesemiconductor material comprising the laser diode that can interferewith light output. To minimize carbon formation, the laser diodestructure can be grown at high pressure within the MOCVD reactor, suchas at atmospheric pressure. Carbon build-up can also be minimized byusing a high V/III ammonia ratio during growth, and by using a slowergrowth rate. Many different precursors can be used during MOCVD growthincluding but not limited to trimethylgallium (TMGa), tiethylgallium(TEGa), trimethylaliminum (TMAl), tiemethylindium (TMIn),Bis(cyclopentadieny)Magnesium (Cp₂Mg), silane (SiH₄) and ammonia (NH₃).

FIG. 1 shows one embodiment the laser diode 10 at an early step in thefabrication process comprising a substrate 12 that can be made of manydifferent materials including but not limited to sapphire, siliconcarbide, with the preferred laser diode 10 being formed on afree-standing GaN substrate or a lateral epitaxial overgrown (LEO) GaNtemplate. N-type contact layer 14 is grown on the substrate 12 thatcomprises a material suitable for spreading current from an n-contact tothe active region. For laser diodes that are formed on conductivesubstrates, the n-contact can be formed on the substrate 12 and currentconducts through the substrate to the active region of the laser diode10. For laser diodes formed on non-conductive substrates or substratesthat do not efficiently spread current, a lateral geometry can be usedfor contacting the device. As further described below for lateralgeometry, the laser diode 10 can be etched to form a mesa in the contactlayer 14 and the n-contact is deposited on the contact layer mesa.Current spreads from the contact, through the n-type layer 14 and to thelaser diode's active region.

It is also understood that laser diodes formed on a conductive substratecan effectively spread current without the n-type contact layer 14, andthat these embodiments can be arranged without the contact layer 14. Itis further understood that other embodiments of the present inventioncan have the substrate removed, and in those embodiments contact can bemade directly to the epitaxial layers.

The n-type contact layer 14 can be made of many differentelements/materials doped with different elements in different densities.The preferred contact layer 14 comprises GaN doped with silicon (Si)having a doping density being between 1E17 to 1E19 cm⁻³. The contactlayer 14 is normally grown at high temperature (e.g. 1000 to 1100° C.)and the preferred growth rate is 1.5 to 3 μm/hr. The preferred MOCVDgrowth carrier gas is hydrogen with a 50% H₂/50% N₂ subflow.

FIG. 2 shows the laser diode 10 after additional growth steps to form acompliance layer 16 on the contact layer 14. The compliance layer isincluded to allow waveguiding elements to be grown with higher index ofrefraction, which in turn allows for the threshold current of laseroperation to be lower. As described below, the waveguiding elements ofsome embodiments of the laser diode according to the present inventioncomprise waveguiding epitaxial layers that can be arranged in manydifferent ways and can comprise many different structures. In oneembodiment according to the present invention, the waveguiding epitaxiallayers comprise a strained layer superlattice (SLS) made of AlGaN/GaN.The higher the Al content of the SLS the higher the index of refractionand the more efficient the guiding of active region light. Without thecompliance layer, however, a good quality SLS may not be reliablyformed. The strain between the contact layer 14 and the SLS structurecould cause the high Al content layers to degrade or crack.

To reduce the danger of degradation and cracking, the compliance layer16 includes one or more elements that help reduce the strain between thecontact layer 14 and SLS. In the one embodiment according to the presentinvention, the compliance layer 16 can comprise n-type In_(x)Ga_(1-x)Nlayer grown by with Si doping. The In element in the compliance layerprovides for the strain relief, with the desirable In composition (x)being between 0.08 and 0.12. It is understood that other compositions of(x) are also acceptable such as 0 to 0.08 and 0.12 to 0.20 or more. Manydifferent Si doping densities can also be used with a suitable dopingdensity being between 1E17 to 1E19 cm⁻³. The preferred MOCVD growthcarrier gas is nitrogen which assists in higher In incorporation, andthe subflow can be 100% N₂.

Because of the In element in the compliance layer, however, thecompliance layer is typically grown at temperatures lower than thegrowth temperature of typical AlGaN/GaN layers. This lower temperaturegrowth encourages In incorporation into the material. Typical growthtemperatures for InGaN are in the range of 700 to 1000° C. A, with asuitable growth temperature being 900° C. Subsequent device layers ofAlGaN/GaN can be grown at temperatures in the range of 1050-1100° C.These elevated temperatures present a danger of desorption or burning ofthe In within the compliance layer 16, that can negatively impact theoperation and efficiency of the laser diode 10. To help reduce thedanger, a thin low-temperature (LT) n-type GaN:Si first cap layer 18 canbe grown on the compliance layer to cap and protect the InGaN compliancelayer 16. Growth of GaN at 900° C. is relatively slow but can be usedbecause the cap layer is relatively thin. Growth at this temperaturealso provides the advantage of not significantly damaging or degradingthe In in the InGaN compliance layer. The cap layer 18 protects theInGaN compliance layer 16 during the temperature ramp-up for fastergrowth of subsequent layers.

FIG. 3 shows the laser diode 10 after additional growth steps, with ann-type SLS 20 formed on the cap layer 18 and compliance layer 16, withthe cap layer 18 between the compliance layer 16 and the n-type SLS 20.The n-type SLS 20 can comprise different layers made of many differentmaterials, with a preferred SLS being an n-type Al_(x)Ga_(1-x)N/GaN:Simodulation-doped SLS. The SLS is preferably grown by Si-doping the GaNlayer(s) only. The preferred thickness of the AlGaN and GaN:Si layers isbetween 2-3 nm and the desirable growth rate is between 0.5-2 A/s, whichencourages Al incorporation at high pressure during growth of the AlGaNlayers. The preferred growth carrier gas is hydrogen with 50% H₂/50% N₂subflow and high growth temperature is normally desired (e.g.approximately 1050° C.). The doping density of the GaN layer(s) isbetween 1E17 to 1E19 cm⁻³ and the most desirable

As discussed above, the compliance layer 16 allows for reliably growthof the n-type SLS 20 with a higher Al composition than could be grownwithout the presence of the compliance layer. The higher Al contentallows the n-type SLS 20 to more efficiently reflect light emitted bythe laser diode's active region. The Al_(x)Ga_(1-x)N layer(s) of then-type SLS 20 preferably have an (x) composition between 0.15 and 0.2.Composition (x) in the ranges of 0.1 to 0.15 and 0.2 to 0.3, as well asother compositions are also acceptable. Higher Al compositions, however,can result in lower conductivity due to the increase in the ionizationenergy of the Si dopant. Lower Al compositions can result in poorwave-guiding because of reduced refractive-index of the SLS 20. The SLS20 can have many different thicknesses, with a preferred thickness beingbetween 0.6 to 1.5 μm.

FIG. 4 shows the laser diode 10 with an n-type separate confinementheterostructure (SCH) layer 22 formed on the SLS 20. The SCH layer 22serves as part of the light path to the edges of the laser diode 10 andultimately out the emission edge of the laser diode 10. The light fromthe active region traveling toward the waveguiding elements (n-type SLS20 and the p-type SLS described below) is reflected, and light travelingtoward the laser diode's edges is reflected until stimulated emission isout one of the edges. The n-type SCH layer 22 and p-type SCH layer serveas the primary reflection cavity for this reflected light.

SCH 22 can comprise many different materials doped in differentdensities by different elements. A preferred n-type SCH comprises ann-type GaN grown with Si doping at a doping density between 1E17 to 1E18cm⁻³. The SCH can have many different thicknesses, with the preferredthickness being in the range of 0.07 to 0.15 μm. The preferred growthrate is in the range of 0.5 to 2 A/s to make the top surface of the SCHsmooth and the preferred growth carrier gas is nitrogen with 100% N₂subflow. By keeping the SCH 22 top surface smooth, subsequent layers canbe grown with better quality. For example, the active region can begrown with layers having more uniform thickness such that the activeregion can emit light with a more uniform wavelength.

A thin undoped GaN layer 24 can be included on the n-type SCH, beforegrowth of the multiple quantum well (MQW) active region. By having anundoped intrinsic material adjacent to the MQW active region, dopantsare inhibited from flowing into the active region absent a drivingcurrent. The preferred thickness of the undoped GaN layer 24 is between5-12 nm. The desired growth rate is between 0.3 to 1.0 A/s to make thelayers smooth and the preferred growth carrier gas is nitrogen with a100% N₂ subflow.

FIG. 5 shows the laser diode 10 with an active region 26 formed on theundoped GaN layer 24. The active region can comprise different layersarranged in different ways, with a preferred active region comprising aMOW region that can have different numbers of quantum wells and barrierlayers. The preferred MQW region having three quantum wells. Quantumwells are included for confinement of electrons and holes to encouragerecombination and the resulting light emission. In general, the largerthe number of quantum wells within the MQW region typically results inincreased gain volume. The greater the number of quantum wells in theMQW region also typically results in a higher necessary thresholdvoltage. Three quantum wells allows for a good combination of currentdensity gain with a relatively low threshold voltage.

In one embodiment according to the present invention, MQW active region26 comprises three quantum wells and associated barrier layers in anIn_(x)Ga_(1-x)N/In_(y)Ga_(1-y)N stack. The desirable composition of thequantum wells had (x) between 0.8 to 0.12 and that of the barrier layerhas (y) between 0 to 0.04 for an approximate 405 nm emission wavelength.The desired well width is between 3-5 nm and the desired barrier widthis between 4-8 nm. The preferred growth temperature is relatively low,between 800 to 950° C. to assist in In incorporation and is grown innitrogen carrier gas with 100% N₂ subflow. The desired growth rate isslow, between 0.3 to 0.6 A/s, to make the interfaces smooth and to lowercarbon incorporation into the active region. The preferred way ofterminating the MQW region is by having a last well instead of abarrier.

FIG. 6 shows the laser diode 10 with an electron blocking layer 28formed on the MQW active region 26. The blocking layer 28 comprises amaterial that blocks electrons from passing from the MOW active region26 into the p-type SCH layer (shown in FIG. 7 and described below), butlets holes pass through to from the p-type SCH layer to the MQW activeregion 26. By blocking electrons, the blocking layer encouragesrecombination in the MQW active region 26.

The blocking layer 26 can be made of many different materials doped indifferent ways. A suitable material p-type Al_(x)Ga_(1-x)N achieved byMg doping and a suitable thickness of the layer is between 15-25 nm. Thepreferable method of depositing the layer is by a two step depositionprocess. First, a 1-10 nm AlGaN layer is grown at lower temperature,preferably between 800 to 950° C. This low temperature growth helpsreduce desorption and damage to the In in the MQW active region 26 thatcan be caused by higher temperatures. Second, the remaining blockinglayer is grown at an elevated temperature, preferably between 900 to1050° C. The preferred Al composition (x) of the blocking is between0.15 to 0.25 and the desirable Mg doping concentration is between 7E18and 3E19⁻³. The preferred growth carrier gas during deposition of theblocking layer 28 is nitrogen with a 100% N₂ subflow and the preferredgrowth rate is 1-2 A/s.

FIG. 7 shows the laser diode 10 having p-type separate confinementheterostructure (SCH) layer 30 formed on the blocking layer 28. Thep-type SCH 30 can be made of many different materials doped in manydifferent ways, with a preferred p-type SCH 30 made of p-type GaN grownwith Mg-doping. As described above, the p-type SCH layer 30, along withthe n-type SCH layer 22, serves as the primary light path out of thelaser diode 10 between the waveguiding elements. The preferred Mg dopingdensity of the p-type SCH layer is between 1E18 to 5E19 cm⁻³ and thepreferred thickness range is between 0.07-0.15 μm. The preferred growthrate is between 1-2 A/s to make the layers smooth and the preferredcarrier gas is hydrogen with a 50% H₂/50% N₂ subflow. The growthtemperature for this layer is relatively high such as between 1000-1100°C. to assist uniform Mg diffusion in the p-AlGaN cap layer.

FIG. 8 shows the laser diode 10 with a p-type SLS 32 grown on the p-typeSCH layer 30. The p-type SLS 32 can be made of many different layers andmaterials arranged in different ways but is preferably p-typeAl_(x)GaN/GaN:Mg modulation-doped SLS grown Mg acceptors (dopants) inthe GaN layer only. The preferred thickness of the AlGaN and the GaN:Mglayer is between 2-3 nm and the preferred growth rate is between 0.5-1A/s to assist Al incorporation at high pressure. The preferred growthcarrier gas is hydrogen with a 50% H₂/50% N₂ subflow. Lower growthtemperature between 850-950° C., can be utilized to reduce Insegregation in the underlying active region 26 during the long p-typeSLS growth step at high temperature. The desired acceptor density isbetween 8E18 to 5E19 cm⁻³ the desirable Al composition (x) is between0.15 and 0.2. Compositions (x) in the range 0.1-0.15 and 0.2 and 0.3 arealso acceptable. Higher Al composition results in lower conductivity dueto the increase in the ionization energy of the Mg dopant. Lower Alcomposition results in poor wave-guiding because of reduced differencein refractive-index between the active layer and the cladding layer. Thepreferred p-type SLS 32 thickness is between 0.6-1.5 μm. Thinner SLS canresult in poor waveguiding and thicker SLS can result in cracking due tostrain.

FIG. 9 shows the laser diode 10 capped with a thin p-type contact layer32 that can be made of different materials doped in different ways, butis preferably p-type GaN by means of Mg-doping. The preferred dopingdensity should be relatively high, with one embodiment having a dopingdensity between 1E19 to 5E20 cm⁻³. A doping ramp from low to highconcentration can be utilized instead of a uniform doping concentration.In some embodiments a uniformly doped contact layer 32 that is heavilydoped can be hazy, which can interfere with efficient operation. Byramping up the doping concentration such that the last portion of thecontact layer 32 is highly doped, the contact layer typically will notturn haze. The preferred thickness range for the contact layer isbetween 20-50 nm and a suitable growth rate is between 0.5-2A/s. Thepreferred carrier gas is hydrogen with a 50% H₂/50% N₂ subflow and thegrowth preferably is at relatively high temperature between 1000-1100°C.

The p-GaN cap layer 34 has relatively high doping and optimum thicknessto allow for reduced threshold voltage. The higher the doping and thethinner the cap layer, the lower the Schottky barrier at the contactlayer junction because of electron tunneling. The contact layer 34,however, cannot be too thin because the layer can then experiencenon-uniform doping. The contact layer 34 should have a sufficientthickness to allow for uniform layer doping. The p-type SLS structure 32has lower doping (but high Al content) compared to the p-type cap layer34, to reduce threshold voltage. With higher doping of the SLSstructure, there is a danger that precipitants from the doping material(e.g. Mg) can form that can be resistive. This can result in higheroperating threshold voltage. The lower doping, such as in the rangedescribed above, can provide good quality materials with noprecipitates. This combination of highly doped and optimum thicknessp-type cap layer 34 with lower doped p-type SLS structure 32, canprovide reduced threshold voltage for laser diode 10.

FIG. 10 shows on embodiment of a laser diode 50 having featuresdescribed above to provide for low threshold current and voltageoperation. For ease of illustration the specific layers are not shown indetail, but it is understood that the layers can be arranged as thoseshown in FIGS. 1-9. The laser diode 50 further comprises a p-contact 52on the top surface of the laser diode 30, which will typically be thep-type contact layer 34 shown in FIG. 9 and described above. Thep-contact can comprise many different materials, but is preferablyformed of combinations of nickel, gold and platinum (Ni/Au/Pt) depositedusing know methods such as sputtering. The laser diode 50 furthercomprises an n-contact 54 on the substrate 12 that is shown in the FIGS.1-9 and described above. For laser diode 50 the substrate is conductive,which allows re-contact to be formed directly on the substrate. There-contact can also be made of many different materials with suitablematerials being combinations of titanium and aluminum (Ti/Al). Currentfrom the n-contact 54 flow through the substrate to the laser diode'sactive region. Pad metals can then be included on one or both of the p-and n-contacts 52, 54 and one or both can include an electricalconnection, such as through a wire bond. In other embodiments, then-contact can be directly connected to a submount such as a printedcircuit board (PCB).

It is understood that the laser diode 50 can also operate without asubstrate, with the substrate being removed following growth of thelaser diode. In those embodiments without the substrate the n-contactcan be on other layers such as the n-type contact layer shown as 14 inFIGS. 2-9 and described above, or on other layers.

FIG. 11 shows still another embodiment of a laser diode 60 according tothe present invention with a p-contact 62 and n-contact 64. The upperportion of the laser diode 60 has been etched to form a ridge throughthe upper portion of the laser diode 60. The ridge is arranged toprovide optical and electrical confinement during operation to increaseefficiency. The p-contact is again on the top surface of the laser diode60, which is also the top surface of the ridge 66 and is also the p-typecontact layer 34 (FIG. 9). The laser diode 60 also has a conductivesubstrate that allows for the n-type contact to be formed on thesubstrate. The p- and n-type contacts 62, 64 can be made of the samematerial as contacts 52, 54 shown in FIG. 10 and described above, andpad metals can also be included on one or both of the contacts 62, 64.In other embodiments the n-contact can be contacted through a submountor PDB that the laser diode 60 is mounted to.

FIG. 12 shows a laser diode 70 also comprising a p-contact 72, n-contact74 and a ridge 76, with the p-contact on the top surface of the ridge76. For laser diode 70, however, the substrate is not conductive, and asa result the n-contact 74 cannot be formed on the substrate. Instead, aportion of the laser diode 70 is removed down to the n-type contactlayer (element 14 described above and shown in FIGS. 2-9), such as byetching. A mesa 78 is formed in the n-contact layer for the n-contact74. Current flows from the n-contact 74 through the n-contact layer andto the active region of the laser diode 70. The p- and n-type contactscan be made of the same material as those described above and can bedeposited using the same methods. Pad metals can be included on one orboth of the contacts 72, 74 and electrical connection can be made toboth, such as through wire bonds.

Although the present invention has been described in considerable detailwith reference to certain preferred configurations thereof, otherversions are possible. Therefore, the spirit and scope of the appendedclaims should not be limited to their preferred versions containedtherein.

1. A method for fabricating a Group-III nitride laser diode, comprising:growing an n-type contact layer on a substrate at a temperature within afirst growth temperature range; growing an n-type compliance layer onsaid n-type contact layer at a temperature within a second growthtemperature range that is lower than said first growth temperature;growing a cap layer on said compliance layer at a temperature in saidsecond temperature range to protect said compliance layer fromtemperatures in said first temperature range used to grow subsequentlayers; growing an n-type waveguiding element and on said cap layer at atemperature within said first growth temperature range, said waveguidingelement grown with a higher index of refraction compared to laser diodeswithout said compliance layer; growing an n-type separate confinementheterostructure (SCH) on said n-type waveguiding element; growing anactive region on said n-type SCH; growing a p-type SCH on said activeregion; and growing a p-type waveguiding element on said p-type SCH. 2.The method of claim 1, wherein said active region comprises multiplequantum wells.
 3. The method of claim 2, wherein said active regioncomprises three quantum wells.
 4. The method of claim 1, furthercomprising growing a p-type contact layer on said p-type SLS at atemperature within said first temperature range.
 5. The method of claim1, wherein said n-type SCH, active region and p-type waveguiding elementare grown at a temperature within said second growth temperature range,and said p-type SCH is grown at a temperature within said firsttemperature range.
 6. The method of claim 1, wherein said firsttemperature range is from 1000 to 1100° C.
 7. The method of claim 1,wherein said second temperature range is from 700 to 1000° C.
 8. Themethod of claim 1, wherein said n-type contact layer comprises GaN grownat a rate in the range of 1.5 to 3 μm/hr and with a silicon dopingdensity in the range of 1E17 to 1E19 cm⁻³.
 9. The method of claim 1,wherein said compliance layer comprises In_(x)Ga_(1-x)N wherein x is inthe range of 0 to 0.20 and the Si doping density is in the range of 1E17to 1E19 cm⁻³.
 10. The method of claim 1, wherein said n-type waveguidingelement comprises an n-type Al_(x)Ga_(1-x)N/GaN strained layersuperlattice (SLS).
 11. The method of claim 10, wherein said n-typeAl_(x)Ga_(1-x)N/GaN SLS has x in the range of 0.1 to 0.3 and whereinsaid GaN layers have a Si doping density in the range of 1E17 to 1E19cm⁻³.
 12. The method of claim 1, wherein said n-type SCH comprises GaNgrown at a rate in the range of 0.5 to 2 A/s with a Si doping density inthe range of 1E17 to 1E18 cm⁻³.
 13. The method of claim 1, wherein saidn-type SCH has a thickness in the range of 0.07 to 0.15 μm.
 14. Themethod of claim 1, wherein MQW active region comprises anIn_(x)Ga_(1-x)N/In_(y)Ga_(1-y)N quantum well stack with x in the rangeof 0.8 to 0.12, and y in the range of 0 to 0.04, and a growth rate inthe range of 0.3 to 0.6 A/s.
 15. The method of claim 1, wherein said MQWactive region comprises three quantum wells with surrounding barrierlayers, the width of each said quantum well in the range of 3-5 nm, andeach barrier layer width in the range of 4-8 nm.
 16. The method of claim1, wherein said p-type SCH comprises GaN grown at a rate in the range of1-2 A/s with a Mg doping density of 1E18 to 5E19 cm⁻³ and a thickness inthe range of 0.07 to 0.15 μm.
 17. The method of claim 1, wherein saidp-type waveguiding element comprises a p-type Al_(x)Ga_(1-x)N/GaNstrained layer superlattice (SLS) with x in the range of 0.1 to 0.3 andsaid GaN is Mg doped at a density in the range of 8E18 to 5E19 cm⁻³. 18.The method of claim 17, wherein the growth rate of said p-type SLS is inthe range of 0.5 to 1 A/s.
 19. The method of claim 4, wherein saidp-type contact layer comprises p-type GaN doped with a Mg doping densityin the range of 1E19 to 5E20 cm⁻³, grown at a rate in the range of 0.5to 2 A/s to a thickness in the range of 20 to 50 nm.
 20. The method ofclaim 4, wherein said p-type contact layer is grown with a ramp-up indoping density.
 21. The method of claim 4, wherein said p-type contactlayer is grown with a ramp-up in doping density.
 22. The method of claim1, further comprising growing a p-type electron blocking layer on saidMQW active region at prior to growing said p-type SCH.
 23. The method ofclaim 22, wherein said blocking layer comprises p-type Al_(x)Ga_(1-x)N,where x is in the range of 0.15 to 0.25, having an Mg doping in therange of 7E18 to 3E19 cm⁻³ and a thickness in the range of 15-25 nm. 24.The method of claim 1, wherein the first 1-10 nm of said blocking layeris grown at a temperature within said second growth temperature range,and the remaining of said blocking layer is grown at a temperaturewithin said first growth temperature range.